The present application relates to a display device and more particularly to a display device for driving liquid crystal molecules in lateral field mode.
Lateral field modes for liquid crystal display devices are drawing attention for their capability to provide a wide view angle and high contrast. Of such modes, the fringe field switching mode (FFS) offers improved aperture ratio and transmittance over the in-plane switching (IPS) counterpart.
FIG. 19 is a plan view of major parts illustrating an example of an FFS mode liquid crystal display device. As illustrated in FIG. 19, an FFS mode liquid crystal display device has a plurality of scan lines 203 and a plurality of signal lines 205 disposed in a matrix form on a drive substrate 201. A pixel electrode 209 is provided at each of the intersections between the scan lines 203 and signal lines 205. The pixel electrodes 209 are patterned in the form of comb teeth with a plurality of electrode sections 209a extending along the signal line 205 (or scan line 203).
Further, although not illustrated in FIG. 19, a common electrode 207 is provided below the pixel electrode 209 which is provided on the substrate 201. The common electrode 207 is insulated from the pixel electrode 209 by an insulating film. The common electrode 207 is provided, for example, in the same layer as the scan lines or in a layer above that of the scan and signal lines. The same electrode 207 is disposed at least over the entire surface of a pixel a.
In an FFS mode liquid crystal display device configured as described above, the multi-domain structure adapted to orient liquid crystal molecules m in multiple domains is advantageous for achieving further improved view angle characteristic. In this case, the electrode sections 209a are bent in different directions in the middle along their extension direction as illustrated in FIG. 19 so that each pixel a is divided into two domains. In these domains, the electrode sections 209a extend in different directions. It is preferred in terms of optical characteristics that the two domains be mirror symmetric with respect to the domain boundary portion as a symmetry axis. This allows for the liquid crystal molecules m in the two domains, divided from the single pixel a, to be driven in different rotational directions, thus providing improved view angle characteristic (color shift) for display of shades of gray and white (refer, for example, to U.S. Pat. No. 6,809,789).
It is certainly true that display devices having the above FFS multi-domain structure offer an improved view angle characteristic. However, the following problem has been discovered in such display devices.
If an external pressure (e.g., finger press) is exerted on the display surface of the display device when white is displayed by applying a voltage between the pixel and common electrodes, so-called reverse twisting will occur. This phenomenon causes the liquid crystal molecules in the pixel to rotate in a direction opposite to the direction of the electric field. It has been found that reverse twisting is a contributor to display unevenness (hereinafter referred to as finger press unevenness) and that the display cannot be restored to its original state when left standing.
It is desired to provide a liquid crystal display device having a lateral field multi-domain structure which can completely eliminate finger press unevenness when left standing.